1. Field of the Invention
The present invention relates to an initiator of a photosensitive resin composition for use in photolithography, and more specifically to a photopolymerization initiator containing at least one unsaturated double bond and at least one oxime ester group in the molecule.
2. Description of the Related Art
Photolithograpy is the most widely used method for fabricating liquid crystal display devices and semiconductor devices. A photosensitive composition for use in photolithography is essentially composed of a photoreaction initiator that tends to generate radicals due to its sensitivity to UV light, a crosslinkable compound containing two or more unsaturated double bonds, a polymeric binder that can be developed by an alkaline aqueous solution to form a thin film, a solvent, and one or more additives. A general photolithography process involves the following steps: washing of a glass or silicon substrate; application of a photosensitive material in the form of a solution; vacuum drying and drying prior to exposure for the removal of the solvent; UV exposure through a photomask in an exposure system to form a pattern; removal the unexposed portion using a solution based on an alkaline aqueous solution to form a thin film; and post-development baking at high temperature to improve the heat resistance and durability of the thin film. Of these steps, the exposure step may also be carried out by direct laser scanning or digital exposure using a micromirror or a microlens, instead of the use of the photomask.
Residue remains after every step of the photolithography process to deteriorate the performance of the equipment and to promote the aging of the equipment, thus causing inconvenience that the production lines of the equipment need to be stopped for periodic washing. Particularly, when the thin film is processed at a temperature as high as 180° C. in the post-development baking step, various kinds of volatile substances are adhered to the inner walls of the equipment. Generally, the solvent in the form of a liquid tends to evaporate continuously with the passage of time, leaving no residue behind. There is little possibility that the additives used in very small amounts may leave residue. However, the photopolymerization initiator in the form of a solid is vaporized together with the solvent and is again solidified on the inner walls of the equipment, leaving residue in the form of a gel or powder. The solid residue falls on the substrate in the subsequent steps to cause defects of final devices.
In recent years, attempts to increase the sensitivity of photosensitive materials have been made to achieve improved productivity in the fabrication of devices using photolithography. An increase in the content of a photopolymerization initiator sensitive to UV light in a photosensitive resin composition is considered as the easiest way. However, inevitable residue from the photopolymerization initiator makes the fabrication of the devices difficult to manage, which becomes an obstacle to the improvement of productivity.
Thus, there is a need for a photopolymerization initiator that is not readily vaporized in the post-development baking step, leaving little residue.